J. Chem. Soc., Furuday Trans. 1, 1988, 84(1), 321-330 Adsorption and Decomposition of Ammonia on an Fe(1 x 1) Overlayer on an Ru(OO1) Surface with or without Co-adsorbed Oxygen Chikashi Egawa, Kyoichi Sawabe and Yasuhiro Iwasawa* Department of Chemistry, Faculty of Science, University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo 113, Japan The adsorption and decomposition of ammonia on Fe( 1 x 1) overlayers on an Ru(OO1) surface with or without co-adsorbed oxygen have been investigated by means of Auger electron spectroscopy, low-energy electron diffraction and thermal desorption, and also by kinetic studies. The adsorption of ammonia at 420 K on an epitaxial Fe overlayer deposited on an Ru(001) surface gave a newly-ordered ( ~ ' 7 x 2/7) R19" structure over a wide range of Fe coverages from 0.14 to 2 monolayers.The desorption of N, from the ordered adsorbate layers showed a sharp peak at 870 K which was replaced by a peak at 970 K above 0.6 monolayer Fe coverage, showing similarity to the decomposition of surface nitride, Fe,N, observed for Fe single-crystal planes. The decomposition of ammonia on Fe/Ru(OOl) surface proceeded uiu two consecutive reaction steps that were dynamically balanced. The activation energy of the formation of atomic nitrogen varied from 33 to 8 kJ mol-' with an increase in Fe coverage from 0 to 1 monolayer, while the rate of desorption of N, was reduced. Accordingly, the steady-state rate of decomposition of ammonia exhibited an optimum Fe coverage to reach the maximum rate. In contrast, a 4 4 x 2 ) ordered overlayer appeared upon the adsorption of oxygen on an annealed Fe/ Ru(001) surface. The steady-state rate of decomposition of ammonia was enhanced 3.5 times by co-adsorbed oxygen, because nitrogen was effectively activated by surrounding oxygen atoms in c(4 x 2) mixed overlayers.The metals iron and ruthenium are both good catalysts for ammonia synthesis and decomposition. Studies of the adsorption of nitrogen on single-crystal planes of Fe have revealed that the dissociative adsorption of nitrogen takes place preferentially on C, sites present on the ( I 11) surface,' where the activation energy for dissociative adsorption is nearly zero. However, the bonding energies between the metal and nitrogen are similar on the three Fe single-crystal planes, taking the difference in activation energy for dissociation into consideration.This suggests the formation of similar surface nitrides on these different planes by the reconstruction of the topmost atomic layers, where various LEED patterns have been observed. The Fe(ll1) surface has also been demonstrated to be most active for ammonia synthesis,2 because the dissociative adsorption of N, is a rate-limiting step in the reaction. The interaction of ammonia with Fe3 and Ru* surfaces using single crystals and the catalytic decomposition of ammonia on transition metals5-' have also been investigated. The decomposition of ammonia on an Ru(OO1) surface1' takes place at ca. 400 K and proceeds through a reaction mechanism of dynamic balance ; two consecutive reactions, the formation of atomic nitrogen from ammonia and the subsequent desorption of surface nitrogen, are balanced to give the overall reaction rate and the corresponding surface nitrogen coverage at steady state, where no inhibition by hydrogen of the reaction rate or the surface nitrogen coverage was observed.Similar behaviour is also observed on Fe foi1,l' where the 32 1 11-2322 Adsorption and Decomposition of Ammonia on Fe reaction starts at 580 K and the activation energies vary from 200 to 0 kJ mol-1 as the reaction temperature is increased from 640 to 830 K. In contrast to these monometallic surfaces, we have recently employed an Fe/Ru(001) bimetallic system and observed the epitaxial growth of Fe overlayers on an Ru(OO1) surface.l2? l3 By depositing Fe on an Ru(OO1) surface at 420 K (an epitaxial surface), Fe atoms grew in two-dimensional islands, and a commensurate Fe p(1 x 1) structure similar to the hexagonal structure of an f.c.c.Fe( 11 1) surface was formed in the first monolayer. On a commensurate Fe(1 x 1) overlayer at 1 monolayer coverage the bonding energy of CO was found to be strengthened owing to the specific expanded ye overlayer structure, where the nearest-neighbour distance of the Fe overlayer (2.706 A) is very close to that for the (111) surface of f.c.c. Fe,N, although the second Fe monolayer changed to an overlayer structure similar to that of a b.c.c. Fe(ll0) surface, exhibiting extra diffraction spots due to a (6d3 x 6d3) R30" overlayer structure. Annealing the epitaxial surface with Fe coverages below 1 monolayer to 1030K (annealed surface) caused a dispersion of Fe atoms from the islands to the bare Ru surface and led to the formation of a variety of mixed sites composed of Fe and Ru atoms, although upon heating to 900 K the modification of electronic structure occurred with little change in the surface structure of the Fe p( 1 x 1) overlayer. On the contrary, annealing the epitaxial surface with Fe coverages above 1 monolayer to 1030 K induced the diffusion of excess Fe atoms into the Ru substrate and left 1 monolayer of Fe coverage on the surface.In the present paper the bonding states of nitrogen, the local chemical interaction of co-adsorbed oxygen and nitrogen and the effect of oxygen on the ammonia decomposition rate on these epitaxial and annealed surfaces with Fe coverages below 1 monolayer are reported in relation to the exploration of fundamental active factors for heterogeneous catalysis as well as properties of bimetallic surfaces.l3 Experimental Experimental procedures were similar to those described previously. lo, l2 Auger electron spectroscopy (AES) and low-energy electron diffraction (LEED) measurements were carried out using a four-grid retarding-field analyser. Thermal desorption of N, following the adsorption of ammonia and N, partial pressures during the decomposition of ammonia were monitored using a mass filter. Results and Discussion Adsorption of NH, on the Fe/Ru(001) Surface Adsorption of ammonia [lo L (1 L = 1.33 x Pa s)] on Fe-evaporated Ru(001) surfaces (epitaxial surface) at 420 K produced extra diffraction spots in addition to the (1 x 1) spots due to the commensurate Fe p(1 x 1) lattice, as shown in fig. 1 (a).The LEED pattern corresponds to a (2/7 x d 7 ) R19' surface structure, where filled circles represent diffraction spots composed of three domain orientations due to the superstructure and small open circles are derived from multiple scattering between the adsorbate and the substrate lattice. The pattern is different from the diffused p(2 x 2) structure observed with a clean Ru(OO1) surface4*'0 and has not been observed for b.c.c. Fe single-crystal plane^.^ In the adsorption of N, on Fe single-crystal planes, a 4 2 x 2) structure appeared on the Fe( 100) surface, while more complex LEED patterns were observed on Fe( 11 1) and Fe( 110) p1anes.l The adsorption of N, on an Fe( 1 11) surface led to the formation of a series of ordered structures of the form (3 x 3), (d19 x d19) R23.4', (d21 x 2/21)R10.9', (32/3 x 32/3)R30° and (2 x 2), and on Fe(ll0) similar complex LEED patterns such as (2x 3) and (; !) structures were reported.These structures could not be correlated with distinct ranges of surface concentrations, butC . Egawa, K. Sawabe and Y. Iwasawa 323 0 (a 1 (b ) (c) Fig. 1. (a) LEED pattern of the (2/7 x 4 7 ) R19" structure obtained from NH, adsorption on epitaxial Fe overlayers on Ru(001). (b) LEED pattern of the ( 4 3 7 x 2/37) R25" structure observed with NH, adsorption on a 0.5 monolayer Fe/Ru(OOl) surface pre-annealed to 900 K or on an Ru(OO1) surface with 0.5 monolayer of Fe and co-adsorbed oxygen.(c) Surface-structure model of the ( 4 7 x 47)R19" structure. 0, Ru atom; 0, 0, Fe atoms; Q, N atom. were considered to be related to the reconstruction of the topmost atomic layers due to the formation of surface nitrides such as Fe,N. In contrast to Fe single-crystal planes, the ordered structure due to NH, adsorption appeared on the Ru(001) surface with Fe coverages as relatively low as 0.14 monolayer, and was continuously present for the whole range of Fe coverages up to 2 monolayers, which is consistent with the epitaxial growth of an Fe overlayer in two-dimensional commensurate p(1 x 1) islands and also with the results of CO adsorption on epitaxial Fe 0ver1ayers.l~ This is supported by the result that the peak area of N, desorption in the thermal desorption spectra (fig.2) increased almost linearly with surface Fe coverage. An exposure of 10 L is sufficient to saturate the desorption states on the epitaxial surfaces. The surface nitrogen coverage at 1 monolayer of Fe was found to be 0.15, where the calibration was performed using the value of CO desorption from a ( 4 3 x 4 3 ) structure (Oco = 0.33). It is in reasonable agreement with the coverage of 0.14 evaluated from a ( 4 7 x d7) R19" overlayer structure. The ordered structure is due to surface nitrogen atoms, since the disappearance of diffraction spots coincides with the onset of N, desorption, as shown in fig. 2; below Fe coverages of 0.5 monolayer the additional pattern disappeared at 800 K, while above 0.5 monolayer coverage it remained at 900 K.The LEED spots due to the ( d 7 x d7)R19" structure were always as sharp as those for the substrate lattice. This indicates that the Fe overlayer structure is perfectly matched to the substrate Ru lattice structure during the growth of two- dimensional Fe islands [see fig. l(c)]. On the other hand, in accordance with the development of the second Fe overlayer, similar to b.c.c. Fe( 110) structure above 1 monolayer, the extra spots due to the ( 4 7 x 47)R19" nitrogen overlayer became diffused, but were sharpened by annealing to 800 K. Fig. 2 shows a series of N, thermal desorption spectra obtained from ammonia adsorption on an epitaxial surface at 420 K as a function of Fe coverage. For Fe coverages below 0.5 monolayer the desorption peak at 870 K grew with an increase in Fe coverage.On increasing the coverage to ~ 0 . 5 monolayer, a new desorption peak at 970 K developed in addition to the peak at 870 K, and the former became predominant above 0.7 monolayer. The drastic change in the thermal desorption spectrum at an Fe coverage of ca. 0.5 monolayer is entirely different from the continuous peak shift towards higher temperatures observed with the thermal desorption spectra of C0,13 which may be caused by a change in the electronic structure of the Fe overlayer resulting from the size of the islands.324 Adsorption and Decomposition of Ammonia on Fe - m Do .- z 600 700 800 900 1000 T/K Fig. 2. N, thermal desorption spectra from NH, adsorption on epitaxial Fe overlayers on an Ru(001) surface. 10 L exposure; = 8.3 K s-l; Fe coverage: (a) 0.15, (b) 0.5, (c) 0.6, ( d ) 0.75 and (e) 1.5 monolayer. The desorption peaks in each region are characterized by their narrow width (70 K), and exhibit peak maxima at essentially the same temperature regardless of coverage.Therefore, the desorption of N, is likely to be rate-limited by the decomposition of surface nitrides such as ‘Fe4N’, proposed in the adsorption of N, on Fe( 11 1) and (1 10) surfaces,l where the desorption of N, by the decomposition of the surface nitride appeared in the temperature range 850-950 K, depending on the single-crystal plane, while bulk nitrides contributed only to a continuous increase in background at temperatures above 1000 K. Although there are differences in the complex LEED patterns on the (1 lo), (100) and (1 1 1) planes, the metal-nitrogen bonding energies on the three planes become similar through a combination of the activation energies for adsorption with those for desorption.In contrast, a sudden shift in the N, thermal desorption spectra from the Fe overlayer on the Ru(OO1) surface shown in fig. 2 was seen, despite the observation of the same LEED structure for nitrogen atoms over a wide range of Fe coverages on the Ru(OO1) surface. Although the drastic change in the thermal desorption spectra is not easily interpreted at present, the possibility is precluded that the higher-temperature peak may be attributed to the desorption of bulk nitride, because it showed a sharp peak width which is completely different from that on Fe single-crystal surfaces.Moreover, it was observed even below an Fe coverage of 1 monolayer while the lower-temperature peak was still absent, which cannot be interpreted by othe bulk nitride alone. Note that the nearest-neighbour metal-metal distance (2.68 A) at the (1 11) suzface of the f.c.c. Fe4N bulk nitride is close to that on the h.c.p. Ru(OO1) plane (2.706 A). Accordingly, the ( d 7 x d 7 ) R19” surface structure of the nitrogen overlayer can be stabilized without the reconstruction of an Fe p(1 x 1) overlayer, as displayed in fig. 1 (c), where each nitrogen atom occupies threefold hollow h.c.p. sites, binding to the three nearest Fe atoms (filled circles) of the Fe(1 x 1) commensurate overlayer. On this adsorption structure the interaction with Ru atoms in the underlying substrate layer can also occur.In contrast, the (d7 x d 7 ) ordered diffraction pattern was not observed on the surface pre-annealed to 1030 K, where (1 x 1) spots were visible, as in the case of the (2 x 2)C. Egawa, K. Sawabe and Y. Iwasawa 325 ordered structure due to D-CO states (dissociated CO).13 This is interpreted by the dispersion of Fe atoms from islands over the surface, leading to the formation of various mixed sites composed of Fe and Ru atoms. The situation is consistent with a change in the N, thermal desorption spectra from the annealed surface, where a broadening of peak shape and a shift towards lower temperatures were observed. On the other hand, following the adsorption of ammonia on the surface pre-annealed to 900 K, an intermediate ordered structure was observed as shown in fig.1 (b). Since the diffraction spots due to the adsorbate structure are restricted on the ring containing the first fundamental spots, an unambiguous assignment could not be made; however, it is probably derived from a ( d 3 7 x 2/37)R25" structure. Since a study of CO adsorption has revealed that such a treatment (annealing to 900 K before adsorption) mainly causes a modification of the electronic structure without any change of the surface structure of the Fe over la ye^,'^ the treatment may induce a change in the attractive and/or repulsive interactions among adsorbates and hence lead to the formation of a differently ordered structure. Decomposition of Ammonia on the Fe/Ru(001) Surface From studies of the steady-state decomposition of ammonia over Ru(OO1)l' and Fe foil'' at pressures around 1 x Pa and temperatures between 400 and 900 K under flow-reactor conditions, the reaction mechanism may be written as follows : ( i ) NH3(g) + N(ads) + 3H(ads) Since both the reaction rate and surface nitrogen coverage at steady state are independent of hydrogen partial pressure, the backward rate in step (i) is not fast and equilibrium is not established. In this mechanism, with an increase in surface nitrogen coverage, the rate of the first step decreases while the rate of step (ii) increases, and thus a steady-state reaction rate and nitrogen coverage are attained under conditions where both reaction rates are equal.The phenomena observed in fig. 2, which are related to the metal-nitrogen bonding energies, were also observed on the annealed surface.Here, step (i) for the formation of nitrogen atoms from ammonia molecules (ca. 20 L) was examined on a surface pre-annealed to 1030 K. The rate of formation of nitrogen atoms was obtained by integrating the area of the desorption peak of nitrogen. The experiments were carried out on surfaces with Fe coverages between 0 and 1 monolayer (less than half the saturation coverage of nitrogen) in order to obtain a rate of ammonia dissociation as near to zero nitrogen coverage as possible. Typical results are shown in fig. 3 for the surface annealed with 0.3 monolayer of Fe. As the adsorption temperature was raised from 300 to 420 K, the integrated desorption area, i.e. the concentration of nitrogen atoms, increased, with the peak maximum shifting towards lower temperatures.This may be interpreted as a first-order desorption with activation energy varying with coverage, which is caused by the formation of various mixed sites of Fe and Ru atoms. Since the formation of surface nitrogen is thought to be an activated process, the rate of formation of surface nitrogen was plotted against the reciprocal temperature in order to evaluate the activation energy. The results are shown as a function of Fe coverage in fig. 4. As the iron coverage increased from 0 to 1 monolayer, the activation energies decreased from 33 to 8 kJ mol-I. The value obtained for the clean Ru(OO1) surface was a little smaller than that (48 kJ mol-') reported in the literat~re.~ Assuming an adsorption equilibrium for ammonia on the surfaces (which is reasonable on the basis326 Adsorption and Decomposition of Ammonia on Fe 700 800 900 Fig.3. N, thermal desorption spectra from NH, adsorption on an annealed Ru(001) surface with 0.3 monolayer Fe coverage. 20 L exposure; adsorption temperature: (a) 300, (b) 320, (c) 335, ( d ) 360, (e) 370, cf) 395 and (g) 420 K. TIK 40 7 - 30 E 0 c, \ Y x 20 5 K .* * m > .- * 2 10 0 \ O\O I . . . . l 0 0.5 1 Fig. 4. Activation energies for the formation of surface nitrogen from ammonia as a function of Fe coverage. Fe coverage/monolayer that the desorption temperature of ammonia is below 250 K), the decrease in the activation energy is partially explained by an increase in the adsorption energy of molecular ammonia on the Fe overlayer, because the desorption temperature (250 K) of ammonia on Fe surfaces3 is higher than that (183 K) on an Ru(OO1) ~urface.~ This indicates that the adsorption energy of ammonia increased by 25 kJ mol-l. In addition, there is a stabilization of the surface nitrogen which reduces the activation energy of the dissociative adsorption of ammonia for Fe overlayers on the Ru(001) surface, as shown in fig.2. As revealed above, both elementary surface reaction rates were affected by an increase in Fe coverage. The rate of decomposition of ammonia at steady state was measured under ca. 5 x Pa of ammonia at three different temperatures (620, 735 and 850 K)C. Egawa, K. Sawabe and Y. Iwasawa 327 0.5 1 Fe coverage/monolayer I " ' " ' " " 0.5 1 Fe coverage/monolayer Fig.5. Decomposition rates of ammonia on Fe/Ru(001) surfaces. Reaction temperature : 0, 0, 620; A, A, 735; 0, ., 850 K. (a) Epitaxial overlayer, (b) overlayer pre-annealed to 1030 K. 735 800 900 1000 T/K Fig. 6. N, desorption curves from the steady state of reaction at 735 K as a function of Fe coverage. (a) 0.11, (b) 0.33, (c) 0.5, ( d ) 0.66 and (e) 0.83 monolayer. as a function of Fe coverage, as shown in fig. 5 for both sets of epitaxial ( a ) and annealed (b) surfaces. Although some scatter was present in the data, the behaviour was similar for both surfaces. The decomposition rates increased with Fe coverage and then decreased. The coverage of Fe giving the maximum decomposition rate shifted to higher values with an increase in the reaction temperature.On the Ru(OO1) surface'' the maximum rate was observed at ca. 600 K, so that the decomposition rate decreased with an increase of the reaction temperature employed in the present study. The presence of an Fe overlayer at low coverages enhanced the overall reaction rates by accelerating the rate of formation of surface nitrogen from ammonia, whereas the rate of desorption of328 Adsorption and Decomposition of Ammonia on Fe (a) ( b ) Fig. 7. (a) LEED pattern of the 44 x 2) structure obtained from 0, adsorption on a 1030 K pre- annealed Fe/Ru(OOl) surface. (b) Surface-structure model of the 44 x 2) structure on 0.75 monolayer Fe overlayer. 0, Fe atom; 0, 0 atom; 0, N atom. N, was reduced owing to the increased stability of the surface nitrogen; the decomposition was suppressed at higher Fe coverages.Since the activation energy for the desorption of N, is much larger than that for the formation of surface nitrogen from ammonia, and moreover since the overall reaction rate is limited by the desorption step, the maximum reaction rates shifted towards higher Fe coverages as the reaction temperature increased. Since the formation of surface nitrogen from ammonia and the desorption of nitrogen are dynamically balanced at steady state, the steady-state surface nitrogen coverage, which is related to the degree of stabilization, is expected to increase with Fe coverage, as is demonstrated in fig. 6. Moreover, the effect of varying the partial pressure of hydrogen on the reaction rate was not observed for PH,/PNH, ratios up to 2, and surface hydrogen coverage at steady state was negligible (below the detection limit of coverage, similar to the results of ammonia decomposition on Ru(OO1)" and Fell surfaces.Decomposition of Ammonia on an Fe/Ru(001) Surface with Co-adsorbed Oxygen The adsorption of oxygen molecules (3.6 L) at 420 K on a surface pre-annealed to 1030 K with Fe coverages > 0.5 monolayer gave sharp, ordered extra diffraction spots as shown in fig. 7(a). It corresponds to a c(4 x 2) ordered structure on the h.c.p. (001) surface as depicted in fig. 7(b), where atomic oxygen is located on threefold hollow sites similar to a p(2 x 2) oxygen structure on a clean Ru(OO1) surface.14 The surface oxygen coverages for both surface structures are estimated to be 0.25.The subsequent adsorption of ammonia on the oxygen-preadsorbed surface at 420 K did not change the extra spots arising from the c(4 x 2) structure. On the other hand, following the adsorption of O,, the p(2 x 2) LEED pattern, which was sharp on a clean Ru(OO1) surface, became streaky and diffused on the epitaxial surface as the Fe coverage was increased. Subsequent exposure to ammonia produced the ( d 7 x 2/7)R19" LEED pattern, and an intermediate LEED pattern as shown in fig. 1 (b), probably with a (d37 x 437) R25" structure, appeared for the surface with 0.5 monolayer Fe coverage. Taking the island growth of the Fe overlayer into consideration, these results indicate that oxygen atoms preferentially adsorbed on the Ru patches and surface nitrides on bare Fe domains uncovered by oxygen coexist with disordered overlayers of atomic oxygen.Consistently, the sticking coefficient of 0, on an Fe( 1 1 0)15 decreases to 0.1 above 0.1 monolayer coverage, whereas a relatively constant value (0.6-0.8) is reported on Ru(001).16 The decomposition of ammonia on a surface with pre-adsorbed oxygen has also been investigated. The effects of co-adsorbed oxygen on the desorption of N, at both epitaxialC. Egawa, K. Sawabe and Y. Iwasawa 329 600 700 800 900 1000 TI K Fig. 8. N, thermal desorption spectra from NH, adsorption on modified Fe/Ru(OOl) surface with Fe coverage of 0.75 monolayer. 10 langmuir NH, exposure, /? = 8.3 K s-l; (a) surface annealed to 1030 K : (b) surface pre-adsorbed with 0,, followed by H, reduction; (c) surface annealed to 1030 K and pre-adsorbed with 0, followed by H, reduction.(6) and annealed (c) surfaces are shown in fig. 8. For comparison with steady-state decomposition of ammonia, the adsorption of oxygen (3.6 L) on the epitaxial and annealed surfaces was followed by H, treatment (2.7 x lop5 Pa at 900 K for 3 min) before NH, adsorption to remove excess oxygen and to make the surface equivalent to steady-state reaction conditions. From AES measurements the surface oxygen coverage on both (b) and (c) surfaces was estimated to be 0.2. The temperature for the maximum desorption of N, was successively lowered in the order ( a ) (pre-annealed and without pre-adsorbed oxygen), ( b ) and (c), with a concomitant reduction in surface nitrogen coverage. This indicates that the bonding energy of the nitrogen atom was reduced by 30 kJ mol-' by the co-existence of oxygen atoms.The trend of lowering of desorption temperatures was closely related to the change in the rate of decomposition of ammonia at steady state on these surfaces. The relative decomposition rates at 735-780 K were observed to be (a):(b):(c) = 1 :2.5:3.5. This is consistent with the result that the desorption of nitrogen is a rate-limiting step in the decomposition of ammonia.'O As shown in fig. 8, the enhancement of nitrogen desorption by the co-adsorbed oxygen was most effective on surface (c), where the ordered c(4 x 2) structure was observed for the mixed overlayer of nitrogen and oxygen. The ordered structure disappeared in accordance with the desorption of N,.In contrast, p(1 x 1) spots due to the substrate lattice were only observed with high background intensity on surface (b), which indicates a disordered structure for the oxygen-nitrogen mixed overlayer. Accordingly, it is likely that the nitrogen atom occupying the central position of a c(4 x 2) unit structure is effectively activated by the surrounding co-adsorbed oxygen atoms, as 2hown in fig. 7 (b), where the distance between nitrogen and oxygen atoms is 4.69 or 5.41 A. This separation may be a reasonable one to provide the interaction between neighbouring adatoms. Thus in the c(4 x 2) structure the co-adsorbed oxygen atoms may electronically modify the Fe overlayer and also activate the surface nitrogen, resulting in an enhancement of the rate of decomposition of ammonia.330 Adsorption and Decomposition of Ammonia on Fe Conclusions The results obtained in this study are summarized as follows.(1) Adsorption of ammonia at 420 K on epitaxial Fe overlayers on an Ru(OO1) surface produced an ordered ( 4 7 x 2/7)R19' structure over the range of Fe coverage from 0.14 to 2 monolayer owing to surface nitrides similar to Fe,N proposed in Fe single-crystal surfaces. (2) The surface nitride was desorbed.at 870 K below an Fe coverage of 0.5 monolayer, while this takes place above 970 K on the surface with coverage above 0.6 monolayer. (3) The activation energy for the formation of surface nitride from ammonia decreased from 33 to 8 kJ mol-1 with an increase of Fe coverage from 0 to 1 monolayer. (4) The rate of formation of surface nitrogen from ammonia increased with increasing Fe coverage, while that of N, desorption decreased.As a result, the maximum rate was achieved at an optimum Fe coverage. (5) Adsorption of 0, on an annealed Ru(OO1) surface with an Fe coverage >0.5 monolayer led to a c(4 x 2) ordered structure. Subsequent ammonia adsorption produced a c(4 x 2) mixed overlayer (nitrogen and oxygen) structure. (6) The rate of ammonia decomposition was enhanced 3.5 times by co-adsorbed oxygen in the c(4 x 2) mixed structure. References 1 F. Bozso, G. Ertl, M. Grunze and M. Weiss, J. Catal., 1977, 49, 18; F. Bozso, G. Ertl and M. Weiss, 2 N. D. Spencer, R. C. Schoonmaker and G. A. Somorjai, J. Catal., 1982, 74, 129. 3 M. Grunze, F. BOZSO, G. Ertl and M. Weiss, Appl. Surf. Sci., 1978, 1, 241; M. Weiss, G. Ertl and 4 L. R. Danielson, M. J. Dresser, E. E. Donaldson and J. T. Dickinson, Surf. Sci., 1978, 71, 599. 5 J. J. Vajo, W. Tsai and W. H. Weinberg, J. Phys. 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